Shrestha Basu Mallick

Ultrathin crystalline-silicon solar cells with embedded photonic crystals

Photonic crystals (PCs) can be used to trap light in thin-film solar cells to increase optical absorption. We fabricated ultrathin c-Si solar cells whose active layer was patterned into a two-dimensional PC with a square lattice of 450 nm diameter holes spaced at a period of 750 nm. The PC couples incident light into quasiguided modes and can be engineered to increase coupling and thus optimize optical absorption. Both short-circuit current and external quantum efficiency measurements show an enhancement in optical absorption, especially at longer wavelengths. Scanning photocurrent maps confirm the improved optical absorption in the PC regions.

Monolithic Photonic Crystals

A method for making monolithic 2-D silicon photonic crystals is introduced. We demonstrate a structure with broad-band reflectivity (> 90% from 1420 to 1520 nm), and a structure with a sharp (3.5 nm) reflection minimum.

A Large-Area High-Reflectivity Broadband Monolithic Single-Crystal-Silicon Photonic Crystal Mirror MEMS Scanner With Low Dependence on Incident Angle and Polarization

In this paper, we introduce a single-axis resonant combdrive microelectromechanical systems (MEMS) scanner with a large-area highly reflective broadband monolithic single-crystal-silicon (SCS) photonic crystal (PC) mirror. PC mirrors can be made from a single monolithic piece of silicon through alternate steps of etching and oxidation. This process allows the fabrication of a stress-free PC reflector in SCS with better optical flatness than deposited films such as polysilicon slabs on low-index oxide. PC mirrors can be made in IR transparent dielectric material and can achieve high reflectivity over a broad wavelength range. PC reflectors have several advantages over other mirror technologies. They can tolerate much higher processing temperatures and higher incident optical powers as well as operate in more corrosive environments than metals. Compared to multilayer dielectric stacks, PC mirrors allow for simpler process integration, thus making them highly compatible with CMOS and MEMS processing. In this paper, we fabricate a PC mirror MEMS scanner in SCS without any deposited films. Our PC mirrors show broadband high reflectivity in the wavelength range from 1550 to 1600 nm, and very low angular and polarization dependence over this same range. The single-axis MEMS scanners are fabricated on silicon-on-insulator (SOI) wafers with the PC mirrors also fabricated in the SOI device layer. The scanners are actuated by electrostatic comb drives on resonance. Dynamic deflection measurements show that the scanners achieve 22deg total scan angle with an input square wave of 67 V and have a resonance frequency of 2.13 kHz.

Multilayered Monolithic Silicon Photonic Crystals

In this letter, we describe the fabrication of double-layered self-aligned silicon photonic crystals (PCs) using directional and isotropic etches. The double-layer PCs represent a step towards practical 3-D PCs based on standard Si processing, and they have several advantageous characteristics compared to single-layer PCs, including higher reflectivity and sharper resonances, as demonstrated both experimentally and by finite-difference time-domain (FDTD) simulations.

Double-layered monolithic silicon photonic crystals

Double-layered, self-aligned, silicon photonic crystals are fabricated using directional and isotropic etches - a potential step towards 3D PCs. One structure shows sharper resonances compared to corresponding single layer structure. Another shows high, broadband reflectivity.

Optimal light trapping in ultra-thin photonic crystal crystalline silicon solar cells

Using thin films of crystalline silicon to make solar cells reduces the cost by reducing the amount of material needed and allowing poorer quality material with shorter carrier diffusion lengths to be used. However, the indirect band gap of silicon requires that a light trapping approach be used to maximize optical absorption. Here, a photonic crystal (PC) based approach is used to maximize solar light harvesting in a 400 nm-thick silicon layer by tuning the coupling strength of incident radiation to quasiguided modes over a broad spectral range. The structure consists of a double layer PC. We show an enhancement of maximum achievable photocurrent density from 7.1 mA/cm2 for an unstructured film to 21.8 mA/cm2 for a structured film for normal incidence. This photocurrent density value approaches the limit of 26.5 mA/cm2, obtained using the Yablonovitch light trapping limit for the same volume of active material.

Large-area monolithic photonic crystal mirrors with high reflectivity in the 1250–1650nm band patterned by optical lithography

This paper describes large area (500 mum - 500 mum) monolithic 2-D photonic crystals (PC) for applications as high-reflectivity, broad-band mirrors in the near-IR (infra-red) spectrum. These large PC mirrors were patterned using an ASM-L i-line stepper to achieve minimum feature sizes of less than 100 nm. The reflectivity spectrum of the mirrors show that the high reflectivity (>90%) bands can be shifted in wavelength by varying the hole sizes of the photonic crystal to cover the 400 nm near-IR band from 1250 nm-1650 nm.

Large-area high-reflectivity broadband monolithic silicon photonic crystal mirror MEMS scanner

In this paper we introduce a MEMS scanner with a monolithic silicon 2-D photonic crystal (PC) mirror with broad-band high reflectivity (>90%) in the 1550 nm wavelength band. The photonic crystal is generated as an integrated part of the MEMS scanner fabrication process, resulting in a monolithic structure with a mirror that is ultra-flat with ~lambda/100 flatness over a 500 mum times 500 mum area. The reflectance spectrum shows that the PC mirror has a high reflectivity (>90%) band from 1520 nm-1620 nm. The scanner has a scan range of 22 degrees at an input square wave of 67 V with a resonance frequency of 2.13 kHz.